1. Field of the Invention
The present invention relates to a wavelength-tunable light source apparatus using active mode locking that employs cross gain modulation.
2. Description of the Related Art
In recent years, light sources in which wavelengths can be changed at high-speed wavelength sweep rates on the order of several hundred kHz to several MHz in a wide wavelength sweep width range of several hundred nanometers have been researched and developed.
One of them is a dispersion tuning technique based on active mode locking using wavelength dispersion of refractive index in a resonator (hereinafter also referred to simply as “dispersion”) is disclosed in S. Yamashita, et al., Opt. Exp. Vol. 14, pp. 9299-9306 (2006) (hereinafter referred to as “Non Patent Literature 1”).
The technique disclosed in Non Patent Literature 1 utilizes the fact that free spectral range (FSR) (free frequency space) varies with a wavelength when dispersion is present in a resonator.
Specifically, a technique is disclosed for changing an oscillation wavelength by changing a modulation frequency by utilizing the fact that a modulation frequency varies with a central wavelength at which a laser oscillates to directly modulate an electric current to be injected into a semiconductor optical amplifier (SOA) forming a laser and to obtain active mode locking.
Proceedings of the Institute of Electronics, Information and Communication Engineers (IEICE) Electronics Society Conference 2009, C-4-14 (hereinafter referred to as “Non Patent Literature 2”) disclose the use of cross-gain modulation employing external light, not using modulation of an electric current to be injected into a gain medium (SOA), to obtain active mode locking.
With the above-described dispersion tuning, because a wavelength tuning operation uses differences of mode locking frequency, in terms of stability of each varying laser oscillation frequency to the time axis, determination accuracy in a modulation frequency is important. Accordingly, a modulation method with reduced temporal variation (displacement) or fluctuation in a signal (hereinafter also referred to as “jitter”) is desired.
The technique of Non Patent Literature 1 modulates an electric current for driving a gain medium, whereas that of Non Patent Literature 2 performs modulation using light modulated from continuous light generated by a semiconductor laser (laser diode (LD)) by an electro-optic modulator (EOM).
However, the techniques disclosed in the above Non Patent Literatures are a modulation method by which the shape of an electric current waveform subjected to electrical modulation is mirrored, so it is difficult to reduce jitter occurring in generation of an electric current and thus further improvement is required.